Vascular filters are well known in the art. They are generally used for filtering blood in a patient's vessels, such as but not exclusively the inferior vena cava (IVC). Such filters may be implanted substantially permanently, for example for treating deep vein thrombosis, or may be temporarily left in a patient for instance during surgery or during the course of a temporary ailment.
A common and advantageous design of filter has a conical shape, formed of a plurality of filter legs expanding outwardly from a central hub. The filter, which can conveniently be deployed by endoluminal administration typically through the jugular or femoral vein, is oriented with its narrowing taper along the direction of blood flow such that the wide end of the filter is upstream of the hub. Blood clots and other debris are caught by the filter legs, where blood clots may dissolve by natural clot lysing. The conical form of the filter is naturally biased into the open configuration, not only by inherent resiliency of the filter structure but also by blood pressure. The filter can be retrieved by withdrawing the filter back into a retrieval catheter. For this purpose it is known to have a retrieval device, such as a hook, at the hub end of the filter.
A problem can occur with such filters when in situ in that they can tilt relative to the vessel to an extent which results in loss of good coupling of the filter to the vessel wall, leading to leakage of unfiltered blood around the filter and, importantly, to any retrieval element such as a hook contacting the vessel wall and becoming embedded in the vessel wall as a result of endothelialisation. When this occurs it becomes difficult if not impossible to remove the filter by an endoluminal procedure, resulting in the need for open surgery.
Regulatory authorities, such as the United States Food and Drug Administration (FDA) prescribe required performance parameters such as a maximum permissible tilt angle and maximum amount of tenting of implantable vascular filters and the like. It is known for such purposes to have filter stabilisation devices incorporated in the filter assembly for keeping the filter precisely aligned in the vessel, that is with the hub positioned centrally in the vessel. Some designs of stabilisation devices can become embedded in the vessel wall by endothelialisation and hence become difficult to remove, while other designs involve the use of long lengths of wire or thread which can become entangled with the legs of the filter, leading to improper deployment of the filter.
Some examples of implantable vascular filters are disclosed in U.S. Pat. No. 8,062,326, US 2006/0203769, U.S. Pat. No. 6,231,589, US 2010/0049238, U.S. Pat. Nos. 4,832,055, 7,896,898 and 4,425,908.